A calorimetric assay was developed for quantitating Bacillus spp cytotoxic effect on VERO cells. The assay was conducted by incubating VERO cells with sterile bacterial supernatant in microliter plates for 2h and 24h. After exposure formazan was added, and 4h later solubilizer was added. After 16h, the plates were analyzed with an ELISA microplate reader at 570 nm. Absorption of formazan was directly related to viable cell number and their growth rate. This assay detected an effect of B.cereus after 2 hours of exposure which was stronger after 24h of exposure. Preincubation with human serum seemed to have it generally protective effect on the viability of the cells, but no difference was found between the effect of serum from exposed and unexposed healthy humans. B.pumilus showed significantly higher readings compared to B.cereus indicating a less cytotoxic effect of this latter Bacillus spp. In conclusion, this assay appears to give a good and reliable estimate of the cytotoxic effect when used with B.cereus. The additional experiments with B.pumilus further confirms that finding.